Sledge-based seabed benthic mapping using a robust underwater laser profiler.
The video clip below shows a short segment of an underwater benthic habitat mapping "ground truth" video survey using a Savante Underwater Laser Profiler and the moment the entire system overturns following a collision with a seafloor boulder. An incident common in benthic survey. The underwater laser profiler survives.
Towed-sledge based "ground-truth" habitat mapping surveys are often conducted for reasons of cost and simplicity and at velocities of up to 2knots along the seafloor. Vehicles are often lowered by rope harness and a basic umbilical from fishing vessels of opportunity. Long, linear transects of the seafloor are conducted and guided using simple compass, GPS navigational tools and actually *very little* else. Surveys of vast areas of the seabed have been made possible through the economics of this low-cost, effective and very fast means of survey.
Limitations on benthic sledge survey sensing
Video cameras and solid state LED lighting, whilst basic tools, are typically the only sensors robust enough that can be reliably deployed aboard these vehicles. They offer environmental researchers vital insight into the diversity, distribution and populations within the benthic environment. Getting video-footage close to the seabed has a practical advantage - it enables the volume of water between subject matter and the video cameras to be kept to an absolute minimum. When operating in littoral and estuarine waters, surveys can often be subject to high levels of turbidity; close-range inspection offers a mobile and realistic approach to acquiring video in these difficult-to-survey areas.
The problem with visual inspection is that it very difficult to acquire measurements using footage from a single low-cost video camera towed at speed. Further, whilst the cameras aboard a towed sledge offer a fantastic viewpoint of the seabed, the presence of boulders and cobbles in the benthic environment often leads to incidents where the entire sledge can be toppled over. The impulse generated during collisions means that only the hardiest of equipment can be used.
Savante has developed a robust and compact underwater laser profiler which can be synchronized with video camera footage and delivers topographic measurement data from the seabed. Something that was previously impossible.
Diver camera captures laser-equipped seabed sled before passing over sample of Sabellaria worm reef (foreground). Image courtesy of SAMS.
An example of a laser-scanned cross-section topographic profile and the corresponding camera footage at the same location is shown below.
Digital stills image from towed-sledge "ground truth" benthic survey
Figure showing underwater laser-scanned cross-section profile of a number of seabed cobbles
Underwater laser profilers enable measurements to be recorded at enormous speed, exceeding that of a sonar profiler by a factor >1000; in our next article, we'll present the "water-fall" footage from a linear-transect "ground truth" survey.
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